This application seeks to renew funding for studies aimed at generating murine immunodeficiency models. The major goals of the previous application were to generate a murine model of Btk- deficiency and to identify novel genes involved in V(D)J recombination and DNA repair as candidates for gene targeting studies to generate novel SCID models. All goals were successfully achieved. The current application builds on the latter set of studies to further evaluate the potential of V(D)J recombination and DNA repair factors to contribute to immunodeficiencies. V(D)J recombination involves lymphocyte specific activities including the recombination activating gene 1 and 2 (RAG-1 and RAG-2) products as well as generally expressed activities, which include components of the DNA-dependent protein kinase and XRCC4. In studies supported by this application, we have already shown that defects in expression of such genes can lead to severe combined immune deficiencies in mice. In our proposed studies, we aim to characterize the novel immunodeficient phenotype of XRCC4 deficient mice and to continue ongoing efforts to generate murine models of several additional DNA repair genes potentially required for normal immune function. We will also use several different approaches to continue to identify novel genes involved in V(D)J recombination/DNA repair and to evaluate their potential role in immunodeficiencies. Finally, we will employ a rapid approach generated in our laboratory to generate and evaluate mice harboring specific RAG-1 or RAG-2 mutations, some of which are suspected to contribute to complex immunodeficient phenotypes in humans.